1. Field of the Invention
The present invention relates to a piezoelectric vibrating piece, a piezoelectric vibrator, an oscillator, an electronic apparatus and a radio timepiece.
2. Description of the Related Art
In a cellular phone or portable information terminal devices, a piezoelectric vibrator using quartz crystal and so on is used as a time source, a timing source of a control signal, a reference-signal source and so on. Various types of piezoelectric vibrators have been provided, and a piezoelectric vibrator having a so-called tuning-fork type piezoelectric vibrating piece is known as one of the piezoelectric vibrators.
FIG. 16 is a cross-sectional view showing a related-art piezoelectric vibrating piece.
As shown in FIG. 16, a tuning-fork type piezoelectric vibrating piece 200 includes a pair of vibrating arm portions 210 and 211 arranged in parallel and a base portion (not shown) supporting base end portions of the pair of vibrating arm portions 210 and 211. Additionally, an electrode film is formed on an outer surface of the piezoelectric vibrating piece 200, and the pair of vibrating arm portions 210 and 211 can be vibrated in a direction moving close to or away from each other at a predetermined resonant frequency when a voltage is applied to the electrode film.
Incidentally, as apparatuses on which the piezoelectric vibrator is mounted are becoming small in size in recent years, the piezoelectric vibrating piece 200 is also desired to be small in size. However, for example, when the width of the vibrating arm portions 210 and 211 is narrowed, the forming width of the electrode film formed on the vibrating arm portions 210 and 211 is also narrowed, as a result, an equivalent series resistance value (crystal impedance (CI) value) is increased and accuracy of an output signal is deteriorated.
In response to the above, a structure in which groove portions 212 are formed on both main surfaces of the vibrating arm portions 210 and 211 by etching processing as shown in FIG. 16 is known. According to the structure, excitation electrodes (not shown) to be paired face to each other on side surfaces of the groove portions 212, therefore, an electric field can be acted in the facing direction efficiently. Accordingly, even when the width of the vibrating arm portions 210 and 211 is narrowed, the electric field efficiency can be increased and miniaturization can be realized while maintaining a resonant frequency F.
An example of the related art includes JP-A 2009-81520.
However, in the case where the groove portions 212 are formed in the vibrating arm portions 210 and 211 of the piezoelectric vibrating piece 200, rigidity of the vibrating arm portions 210 and 211 is reduced. In particular, when the groove portions are formed in the vicinity of a connecting portion between the base end portions of the vibrating arm portions 210, 211 and the base portion, it is difficult to obtain sufficient strength of the vibrating arm portions 210 and 211, and stress concentration may occur at the portion. Accordingly, when an external impact and the like are applied to the piezoelectric vibrating piece 200, there is a danger that a fracture and so on occur from the vicinity of the connecting portion between the base end portions of the vibrating arm portions 210, 211 and the base portion. That is, there is a problem that the rigidity of the vibration arm portions is reduced when the groove portions 212 are provided in the vibrating arm portions 210 and 211.
Moreover, in the case where the groove portions 212 are formed at the vibrating arm portions 210 and 211, there is the following problem in addition to the above “problem that the rigidity of the vibrating arm portions is reduced”. That is, the groove portions 212 are formed by performing wet etching to a wafer made of quartz crystal or the like by using a mask pattern. The material of quartz crystal or the like has given crystal axes, having a property in which etching speed differs according to the crystal-axis direction. Such property is also called “etching anisotropy”. Specifically, it is known that the etching speed is reduced in the order of Z axis, +X axis, −X axis and Y axis in respective crystal axes (X axis, Y axis and Z axis) of quartz crystal. As the material has the “etching anisotropy”, it is known that the cross-sectional shape of the groove portion 212 obtained after the etching is not a simple rectangle but a shape having inclined surfaces as shown in FIG. 16.
Here, a portion of the inclined surfaces in the groove portion 212 as shown in FIG. 16 is called an “etching residue 213”. Normally, when the tuning-fork type piezoelectric vibrating piece 200 is formed, the wafer is cut from a rude ore of quartz crystal so that the Z axis of the crystal axes approximately corresponds to a thickness direction of the piezoelectric vibrating piece 200, the Y axis approximately corresponds to a length direction of the piezoelectric vibrating piece 200 and the X axis approximately corresponds to a width direction of the piezoelectric vibrating piece 200 for obtaining a desired outer shape by the etching processing. At the time of forming the groove portion 212, the etching residue 213 is generated on the side surface of the groove portion 212 being affected by the etching anisotropy described above. Specifically, the delay occurs in the etching speed as coming from the +X axis direction side toward the −X axis direction side, therefore, a −X axis side surface 212a positioned on the −X axis side in side surfaces facing to each other in the X-axis direction in the groove portion 212 will be an inclined surface gradually inclining toward the +X-axis direction as coming toward a bottom portion of the groove portion 212. Then, the inclined portion will be the etching residue 213 described above. Note that a +X axis side surface 212b positioned on the +X axis side will be a side surface parallel to the Z-axis direction (the side surface not having the etching residue 213). The etching residues are formed not only on the groove portions 212 but also on side surfaces of the vibrating arm portions 210 and 211. In FIG. 16, the inclinations (etching residues) are formed on side surfaces on the +X axis side in both side surfaces of the vibrating arm portions 210 and 211.
When the etching residue 213 is generated in the groove portion 212, shapes are different on both sides with respect to a center line O′ dividing each of the vibrating arm portions 210 and 211 in half in the X-axis direction, and the weight balance is lost. As a result, there are problems that variation of drive-level characteristics of the vibrating arm portions 210 and 211 (behavior of the resonant frequency F with respect to a voltage to be applied on the piezoelectric vibrating piece 200), increase of the CI value due to vibration leakage and so on occur. That is, when the groove portions 212 are provided on the vibrating arm portions 210 and 211, there is also “a problem that the vibration balance is lost” due to the etching residue 213 in addition to the “problem that the rigidity of the vibrating arm portions is reduced”.